The present invention relates to RF power amplifiers and more particularly to compensating for unwanted phase modulation thereof.
The U.S. Pat. No. 4,580,111, to H. I. Swanson, discloses an amplitude modulator which generates an amplitude modulated carrier signal by selectively turning on and off a plurality of RF amplifiers in a digital manner to produce amplitude modulation. The drive for all of the amplifiers is obtained from a common RF driver. These RF amplifiers are either fully on or fully off, each RF amplifier exhibiting a characteristic in that its input circuit has input impedance which is greater when fully on than that when it is fully off. This is due, at least in part, to a capacitance change which takes place during the operation. Thus, when such an amplifier is fully off, there is no Miller capacitance. The Miller capacitance is a function of the input-to-output capacitance and gain and is present when such an amplifier is on. Consequently, when the amplifier is off there is no gain and therefore no Miller capacitance. This change in the input impedance for each amplifier will phase modulate all of the amplifiers because they are driven from a common driver.
This unwanted phase modulation will modulate the RF carrier signal and the AM modulated signal when such RF amplifiers are employed in an AM broadcasting system. This will require wider frequency bandwidth. Also, there is a potential for interference with neighboring stations. In AM stereo broadcasting systems employing such RF amplifiers, this unwanted phase modulation may degrade the AM stereo signal resulting in distortion.